This invention relates to devices for the electrostatic perforation of webs made of paper of other materials, particularly of the webs used in the manufacture of ventilated cigarettes, and which may be formed by the paper web which enwraps the tobacco rod to form the envelope or the web of covering material, or "cork", forming the junction band of a cigarette with the respective filter.
Said electrostatic perforating device substantially comprises two opposite electrode-carrying heads being traversed therebetween by a web to be perforated, one of which is provided with a plurality of individual needle-shaped electrodes arranged substantially perpendicularly to the plane of the web and cooperating each with a corresponding individual and co-axially opposite needle-shaped electrode provided on the other head, or with a common plate-shaped electrode provided on said other head.
In the electrostatic perforating devices of the type mentioned above, the electrodes of the two electrode-carrying heads are maintained at different electric potentials and the web advancing at constant speed between the two electrode-carrying heads is perforated by the sparks striking between the individual needle-shaped electrodes of one electrode-carrying head and either the common electrode or the corresponding needle-shaped electrodes of the other electrode-carrying head.
In order to obtain optimum operation of the electrostatic perforating devices of the type described above, the distance between the opposite electrodes of the two electrode-carrying heads must be kept constant within somewhat strict tolerance limits. For this purpose, electrodes made of tungsten or of tungsten-based alloys are used. However, the tips of the individual needle-shaped electrodes wear out quickly, and even with different rates, and the right distance between said electrodes, therefore, must be re-set.
The object of this invention is to provide an electrostatic perforating device of the type described above, wherein the right distance between the electrodes can be re-set periodically, semi-automatically, quickly and exactly with the aid of a calibrated or gauged shim.
This problem is overcome by the invention because the individual needle-shaped electrodes are axially slidably arranged in the respective electrode-carrying head and can be advanced toward the opposite electrode-carrying head by a pushing force and against the action of an opposing force, until their front ends will engage a gauged shim placed between the two electrode-carrying heads, and adapted to establish the right distance between the electrodes of the two electrode-carrying heads.
Preferably, according to an embodiment of the invention, said pushing force is exerted separately on each individual needle-shaped electrode and is obtained specifically by means of a pressurized fluid, for example by means of compressed air. Said opposing force is also exerted preferably separately on each individual needle-shaped electrode and can be constituted, for example, by a frictional resistance tending to maintain the individual needle-shaped electrodes in their positions.
Thus, by merely inserting said gauged shim between the two electrode-carrying heads of the electrostatic perforating device, and then by pushing the individual needle-shaped electrodes, for example by means of said pressurized fluid until their tips will engage said gauged shim, it will be obtained, after said shim has been removed, the exact pre-established distance between the electrodes of said electrode-carrying heads, by means of a periodical, semi-automatic operation for resetting said distance.
When only one electrode-carrying head is provided with needle-shaped electrodes, while the other electrode-carrying head is provided with a single plate-shaped electrode, according to the invention, only the needle-shaped electrodes will be advanced. However, when both the electrode-carrying heads are provided with needle-shaped opposite and co-axial electrodes, the electrodes of both electrode-carrying heads are so designed as to be advanced.
A further object of the invention is to provide an electrostatic perforating device of the type described above, wherein the distance between the electrodes of the two electrode-carrying heads will be kept constant automatically, within pre-established wear limits, during the operation of the device, so as to reduce the frequency of the periodical, semi-automatic operations for resetting the right distance between the needle-shaped electrodes. To achieve this object, the invention provides for an automatic mutual approaching movement of the electrode-carrying heads during the operation of the electrostatic perforating device, through a relative micrometric movement proportional to the increase of the distance between the electrodes, said approaching movement being thus capable of compensating for the wear of said electrodes. This automatic and micrometric mutual approaching movement of the electrode-carrying heads may be either continuous or, preferably, discontinuous and may be obtained either by moving only one electrode-carrying head while the other is kept stationary, or by moving both electrode-carrying heads. The mutual micrometric approaching movement of the electrode-carrying heads to compensate automatically for the wear of the electrodes may be obtained by any suitable means, for example by a cam.